RUNOFF-GENERATED DEBRIS FLOWS IN BURNED AND UNBURNED BASINS, YELLOWSTONE NATIONAL PARK AND CENTRAL IDAHO

Runoff-generated debris flows are common after severe forest fires in mountain environments because of reduced infiltration rates, smooth soil surfaces with abundant erodible fine sediment and ash, and extreme discharges that entrain sediment from low-order channels. After the 1988 Yellowstone fires, brief, intense convective storm precipitation caused widespread surface runoff in severely burned basins, although hydrophobic soils were uncommon. Surface sealing by raindrop compaction on bare, ash-mantled soils likely limited infiltration. Rilling was pervasive on 25-35° headslopes, but only local evidence for thin onslope debris flows was observed. Deep incision and further sediment bulking occurred in channels, but levees, mud coatings, and other evidence for debris-flow conditions was not usually observed above middle reaches of trunk channels. In a sediment budget developed for a 1989 event in a 1.6 km² basin, rilling accounted for 30% of the ~11,500 m³ of total erosion, with most of the remainder from trunk channel incision (58%). Mud and ash from rills provided abundant matrix for the debris flow. A classic boulder lobe contained only 30% of the alluvial fan deposit volume, whereas 47% consisted of gravel-poor “mudflow” facies that likely moved in both precursor surges and tail sections of the flow. Sediment concentrations tended to become more dilute over the duration of single events. This trend was also observed with time after fire, likely due to depletion of available soil-surface fines and soil-surface compaction. Runoff-generated debris flows were most common within 2-3 yr after fire, but occurred for up to 10 yr in basins where revegetation was slow and channels were reloaded by ravel from steep incised channel margins and adjacent slopes. We observed similar postfire runoff-generated debris flows in the southern Idaho batholith area, however, very large debris flows were also caused by an Aug. 22, 2003 thunderstorm on steep, unburned, sparsely vegetated 30-40° south-facing slopes of the Slaughterhouse (3.6 km²), Slim (6 km²), and Deadwood Jim (2 km²) Creek basins. Deep rilling of grussy colluvium produced onslope debris flows that converged on trunk channels, causing deep incision and massive flows that temporarily dammed the Deadwood River. A nearby station received 32 mm of rain, but precipitation intensity over these basins was likely much more intense.